As an optical glass having a refractive index (nd) of about 1.6 and an Abbe's number (νd) of about 60, so-called SK glasses are hitherto known. For example, “1991 Data Book of Glass Composition” (issued by Japan Glass Product Industrial Society, page 91) describes SK16 glass (nd 1.62, νd 60.3).
Since, however, these glasses have a transition temperature that is as high as 660° C., they are not suitable as a glass for precision press-molding. When it is attempted to decrease the glass transition temperature for overcoming the above drawback, there is caused a problem that the glass is deteriorated in weather resistance. When the glass having poor weather resistance is used to make a precision press-molding preform, the surface of the preform is altered with the passage of time, and the surface of an optical element obtained by precision press-molding of the preform is eventually covered with such an altered layer. Further, the surface of an optical element formed of a glass having poor weather resistance is altered with the passage of time as well, and the optical element is inevitably deteriorated in performances to a great extent.
When attempts are made to improve such a glass in weather resistance, there are caused problems that the meltability of the glass is impaired and that the shapeability of the glass in the shaping of a precision press-molding preform from a molten glass is impaired.